1. Field of the Invention
The present invention relates to zoom lenses.
2. Description of the Related Art
Recently, for an image pickup apparatus or a camera (e.g., a video camera or digital still camera) using a solid-state image pickup element, there have been demands for a compact zoom lens having high optical performance.
In a camera of such type, various optical members, such as a low-pass filter and a color correction filter, are disposed between the rearmost lens element and a solid-state image pickup element. Therefore, it has been demanded that a zoom lens for such a camera has a relatively long back focus. Examples of optical systems having a long back focus include a negative lead type zoom lens in which a lens unit having negative refractive power is arranged at the front.
For a camera using a solid-state image pickup element for color images, it has been demanded that a zoom lens has good telecentric characteristics on the image side so as to avoid color shading.
There is known a compact three-unit zoom lens that includes, in order from the object side to the image side, a first lens unit having negative refractive power, a second lens unit having positive refractive power, and a third lens unit having positive refractive power, and is telecentric on the image side (see, e.g., U.S. Pat. Nos. 6,545,819 and 6,498,687).
Also, there is known a three-unit zoom lens of this type in which all lens units move during zooming and includes a cemented lens component in a second lens unit to correct chromatic aberrations (see, e.g., U.S. Patent Application No. 2005/259334 and U.S. Pat. No. 7,215,483).
With recent developments in high-performance image pickup elements, there have been strong demands for a compact and high-performance zoom lens for video cameras, digital cameras, and the like. To achieve a thin image pickup apparatus, a so-called retractable zoom lens in which distances between lens units are reduced in a non-photographing mode is used. Additionally, it is necessary to reduce the thickness of lens units to achieve a thinner image pickup apparatus.
In the three-unit zoom lenses described above, the second lens unit serves as a main magnification-varying lens unit which primarily performs a magnification varying operation. Therefore, to achieve high optical performance throughout the entire zoom range, it is necessary to reduce, in the second lens unit, variations in aberration (in particular, chromatic aberration) during zooming. That is, to reduce the size of the entire zoom lens system and achieve high optical performance throughout the entire zoom range in the three-unit zoom lens, it is important for the second lens unit serving as a main magnification-varying lens unit to provide an appropriate lens configuration. Additionally, it is important to select appropriate materials for forming respective lens elements constituting the second lens unit, thereby reducing variations in chromatic aberration associated with zooming.